Varieties of films have been offered so far in order to enlarge a viewing angle of a liquid crystal display. As a film to enlarge a viewing angle, there have been offered a polarizing plate prepared by adhered a retardation film on a surface of a common polarizing plate of a liquid crystal display, a polarizing plate in which a liquid crystal layer is provided on the polarizer protective film of the polarizing plate or a polarizing plate employing a polarizer protective film having a retardation function, in order to enlarge the viewing angle.
A cellulose triacetate (triacetyl cellulose, TAC) film has been conventionally used for a polarizer protective film of a liquid crystal display. TAC film exhibits high transparency, excellent adhesiveness to a polyvinyl alcohol film which is used as a polarizer and smaller birefringent anisotropy. Accordingly, TAC film has been suitably used as a polarizer protective film, since it gives no undesired retardation effect to a polarizing plate. However, on the contrary, TAC film has been thought to be unsuitable for such a purpose that expansion of viewing angle was attempted by providing a retardation effect to a polarizing plate itself, since, although TAC film has some extent of positive birefringency, it is difficult for a TAC film to have a sufficient retardation effect by stretching the film, due to its too small anisotropy. After that, there developed has been a technique using a material in which retardation effect was provided by changing the acetyl substituent of triacetyl cellulose (for example, refer to Patent Document 1) or a technique in which a film having larger retardation effect was prepared by adding a material providing a retardation effect (for example, refer to Patent Document 2). These techniques have already been commercialized. Thus, it has become possible to provide a retardation effect to a polarizer protective film itself, by suitably stretching a cellulose ester film provided with anisotropy. Specifically, expansion of viewing angle of a VA (vertical alignment) mode liquid crystal display has become possible.
However, thus prepared polarizing plate having a different retardation effect from that of the commonly used TAC film also has the following problems as well as the conventional TAC film. When a polarizing plate laminated of a glass substrate of a liquid crystal display is kept under a higher temperature condition, the polyvinyl alcohol film as a polarizer may shrink and give a stress to the polarizer protective film, resulting in changing the retardation effect of the polarizer protective film. Thus, there may occur a leakage of light in the black image portion of the screen, namely, a frame unevenness (also referred to as “corner mura”) which occurs at peripheral areas near the four sides of a TN (twisted nematic) mode liquid crystal display or a corner unevenness which occurs at the four corners of a VA mode or a IPS (in-plane switching) mode liquid crystal display. Also, a phenomenon in which whole the screen seems whitish (whitish screen problem) may occur, which is due to property changes occurring in the polyvinyl alcohol and TAC caused by the applied heat and humidity.
On the other hand, a cellulose acylate film exhibits optical anisotropy, although it is small. The small optical birefringency of the cellulose acylate film may give an undesired effect on the viewing angle property or color hue of a liquid crystal display. As a method to reduce the optical anisotropy, there disclosed has been a technique to add a material which compensates the anisotropy, namely, a material having negative birefringency, for example, acryl polymer (for example, refer to Patent Document 3) or a styrene polymer (for example, refer to Patent Document 4), in order to control the retardation effect.    Patent Document 1 JP-A No. 2001-188128 (hereinafter, JP-A refers to Japanese Patent Application Publication Open to Public Inspection    Patent Document 2 JP-A No. 2000-111914    Patent Document 3 JP-A No. 2003-12859    Patent Document 4 JP-A No. 2005-105140